Equalization of transmission



April 12, 1927.

M. E. STRIEBY ET AL EQUALIZATION 0F TRANSMISSION Filed March 1926 2 Sheets-Sheet 2 INVENTORS L mom/Ev 4 considered necessary Patented Apr. 12, 1927. f

U'Nl-TED STATES MAURICE E. s'rmEBY AND CHARLES H. FETTEB,. or

smNoEs 'ro AMERICAN TELEPHONE AND TELEGRAPH on NEW YORK.

Applicatiog filed March 27 relates to equalization of larly to the This invention more particu transmission, and

connection of equalized transmission circults in tandem.

In a telephone plant where high quality transmission-is desirable, it is the practice to compensate for the variation of attenua tion of a transmission by associating 'with'the circuit a network so '10 designed asto have a variation of' attenuation with frequency which is complementary to that of the circuit. As the equalizing netless the conditions are maintained. For example, the so-called shunt type of equalizer is a ranged to properly e non-10a ed cable circuit un erconditions such that the cable circuit is terminated at one end in a resistance of 500 ohms and the equalizer is connected-between the other end of the. non-loaded cable circuit and a pure; resistance. In attempting to directly connect the terminals of two independently equalized lines with the equalizers located inthe usual manner, one of the equalizers would be connected directly between two non-loaded cable circuits instead 'ofbeing connected between a non-loaded cable circuit, and a pure resistance.

heretofore to separate the two circuits either bya anceartificial-lin'e or by means of an amplifier inlorder to maintain the impedance con-z ditionfor proper equalization. Under certain conditions this would be uneconomical, 'q'uire additional apparatus. be physically possible to equalizer properl designed to equalize two circuits connecte intandem, this would be impractical because it is desirable to have the circuits connected in tandem in different combinations. Therefore, in accordance with the present invention the equalizers are While it would when the circuits are circuit with frequency v ualize a given- It has therefore been I section of resist:

and in any event would re-' provide a single cated .in Fig PATENT OFFICE. j

NLLLEUBN, NEW JEnsEir, as-

ComraNz'aCo ronazrIoN EeUALIzA'rroN or mmsmssro n v.

,-1926. Serial No. 98,00Q. v

solocated with respect to the circuits directly connected toether at their terminals each of the equalthat izers will be in such relationto the circuits I This result may be as to operate properly.

the equalizers at accomplished by connecting the ends of the lines and then connecting together those ends of the lines at-which the equalizers are located. It has been found that by: connecting the circuits in this manner the conditions for proper equalization are substantially fulfilled for. each line-so that the transmission characteristic of the over-all circuit is substantially as flat as the rately. Measurements. also show that the transmission-equivalent of the over-all-cir- 'cuit is somewhat less than the sum of-the two equalized circuits work used for equalization of transmission;

,Figs. 4 and dare simplified diagrams showing the electrical relations involved in an equalized circuit; .Figs .f.6 and 7 are curves showing the impedance teristics ofa properly terminated line and an-equalizer for equalizing transmission over the line; Fig.8 is a diagram showing the elements of. two independently "equalized lines which are to beconnected in tandem;

Figs. 9 and 10 illustrate methods of con-- necting such lines in tandem which will. not give proper equalization; Figs. 11 and 12 show how independently equalized lines may be indirectly connected through an amslifier .and an artificial line,

13 illustratesan arrangement of equalizersrespectively; an

whereby two independently equalized lines may be directly connected together type of system to which Fig.

sum of the two equalized circuits taken sepa-- 8 Single'equalmed cllcllit; Fig. 3 shows the form of netfrequency charac- I lVhile' the present invention is applicable to many different situations, a typical situation involving the necessity of connecting separately equalized lines in tandem is indi- .;1. In this figure 1, 2 and 3 designate different pick-up stations located at various distances fromfa central switching '12, etc. be equalized (that is, it could be made to station to which they are connected by practice may be connected intandem in various combinations; for example, at 0611321111 times pick-up station 2 may be connected to broadcast transmitter 11; at other times it may beconnected to broadcast transmitter The complete tandem circuit could transmit all frequencies within a given range with substantially uniform-attenuation) by the use of a single equalizer designed for .use with the complete over-all line. This would mean, however, that every time a dilferent combination of circuits were connected in tandem a different equalizer would have to be used. It is therefore desirable that each line be independently equalized and so ar-.

ranged that it may be connected in tandem to any other line without disturbing the equalization of the circuit as a whole.

As has already been stated-it does not follow that because two lines are properly equalized as independent units they will function as a single equalized line when connected together in tandem. In fact, the matter of connecting together such lines in tandem has been a matter of considerable difiiculty.

In order to understand how this difficulty arises it is desirableto consider briefly the fundamental nature of an equalized line and certain of the practical requirements which must be met before proper equalization is possible. Referring to Fig. 2, let us consider a line L on non-loaded cable and of a given length. Assuming that this line is to be used for transmission in a direction from westto east, the line will necessarily be ter minated. in a terminal impedance at the of music and the like to .sending end S. In the practical case 'where the line is being used for the transmission broadcasting stations the sending end will include pick-up apparatus connected to the line terminal through an amplifier. The output imped ance of the amplifier will constitute the termination of the line and in practice the line is terminated at the sending end in an iinpedance which is equivalent toa resistance of 500 ohms, as indicated at S. At the receiving end of the line it must also be ter- 'minate.d in an' impedance of some kind depending on. the nature of the receiving apparatus. .This impedance is represented at R. The transmission equalizer E is then so designed as to equalize the transmission over the line when the equalizer is connected between the line andthe receiving end termination R, with the line L terminated at the sending an equivalent resistance nation being connected in series with an adjusitable resistance 1 izer as illustrated in Fig. 3 is designed to equalize transmission over the line L under the conditions above stated, it will not function properly to equalize the line if thev value of the terminal impedance S is-ma terially changed, and it will only function properly if the terminating impedance R is a pure resistance.

7 When such. an equal- In order to understand why is so let us assume that the'line of Fig. 2'has been equalized under the conditions above stated. The complete circuit is equivalent to the circuit shown in Fig. 4 in which Z repre sents the sending end impedance, Z represents the impedance at the line terminals looking toward the receiving end and ZE represents the combined impedance of the transmission equalizer -E and the receivingtermination B. Let us assume that a voltage 6 is applied in series with the sending end impedance Z If the line has been properly equalized the voltage drop V across the receiving terminals will bear a ratio-to the applied voltage e which is constant with frequency. 7 It may be shown that in a circuit organization'such'as that of Fig". 4 the following relation holds: 7

i im z, sinh 101T coshTa of the receiving termination, the drop V,- is

related to the a'ppliedvoltage e as follows; 1. Z +Z, Combining equations 1 and 2 we have:

. Z A Z +Z ZL siILh Cosh la The condition for equalization is that the term which is the coefficient of e in the above expression must be constant With frequencyl Thisfcoeflicient may be represented inthe form of two factors'as follows:

z; v A 2.... w Z +Z5 Z Sinh 1C+ZE cosh 1d I:

For a given circuit 1 and 04 are constant and'for a given sending termination Z is" change in the value of the 'ance Z will wards the sending equalizer will be 7 assumed where the QOnSt/ant- ZER, pedance of the equalizer and is therefore variable with fre'uency, and similarly the impedance Z loo ing into'the electrically is variable with frequency." Conlongline, sequently both terms of expression 4 are variable. Therefore, if the expression is to be constant with frequency the two terms of Otherwise the two factors would not vary with frequency in accordance with complementary laws. i

It has now been shown why an equalizer will only. properly equalize the line when it is terminated at the opposite end by an invariable impedance. It remains to be shown why the equalizer must be connected; be-

' tween the line to be equalized and a receiving termination which -[is a pure resistance.

Referring to the equivalent circuit of Fig. 5, the impedance looking into the line toend .at the left of the (In the case already the terminal impedance S does not affect the impedance Z so thatit will be the same as the impedance looking from the sending end- W-l'llCll is Z Z is the impedance of; the equalizer E.- This impedance as seen from the receiving terminals is in parallel to the impedance Z looking into the liner Now the impedance looking into theiline varies with frequency in the general relation indicated by the'curve of Fig. 6. The.

- tion be determined hand, varies in a. somewhat. complementary I 7, as shown-1n Fig. 10. Assuming, as would impedance of the equalizer, on the other manner, as shown by the, curve of 'Flg. If the impedance of the parallel com'binafor each frequency in accordance with the formula pedances connected in parallel, the resultant impedance will be found to be substantially constant for all frequencies within a range from say 100 cycles to 5,000 cycles. In other words, the impedance Zens, "looking into the combination of the line and equalizer from the receiving end will be substantially constant within therange of fr uencies with which we are concerned. bviously, in Fig. 5 we may connect a pure'resistance R across the terminals of the circuit without materially fleeting theequal} however, includes the im-' end the equalization will be disturbed,

is evident from the form of the since the line L 'tant end in animpeda'nce of 500 ohms. As

line is electrically long nation -in'Fig.- 9 is reversed by izer E" the'cente r' and equalizer E 'at the sending end. An alternative arrangement that suggestsfor two 1m-.

' sending" impedancelthese being the conditions met with in practice), it will be clear- "L' the conditions for equalization ization. On the other hand, if a terminal Y impedance which varies with frequencybe connected across the circuit at the receiv1ngfor The varying reflection losses, will occur.

second requirement for proper equalization then is that the equalizer muzt be connected between theline and a terminal impedance which is a pure resistance. The actual value -of*the terminal impedance is of less im portance since the only effect of a'change in the value of. the resistance is to change. the ratio of the drop V to the applied voltage 6 without producing; any variationof this ratio with frequency. v

From the above considerations it will be evident that if we have two equalized circuits L and L, as shown in Fig. .8, the two lines cannot be connected in tandem by directlyconnccting thereceiving terminal of the one to the'sending terminal of the other, as shown in Fi 9. This would resultin having one e'qua From the standpoint of the equalization may be disturbed is not. terminated at thedisone terminal. equal zer E izer near the middle of the over-all circuit and the other equalizer near be substantially constant, vb u'tfiit is not neces- I sarily. substantiall constant at 500 'ohms. On "theu'other' han 'from the standpoint of the equalizer, E; insteadofbeing connected between the line L to be equalized and a pufe it is connected between the" line resistance,-

L and the impedance looking into the line i as a matter of .fact, varies with frequency.

L'fwhich is certalnly not a-pu-re "resistance considerations that the two parts of the circuit will not be properly equalized if the sit lacing equalitself I would be tc connect the 1 equalizers across the sending terminal of the one line,

and the receiving terminal of the other line be the case in practice, that the receiving terminatlo'n 1s equivalent'to a ure resistance of 500 ohms and is therefore t e same as the that proper equalization will not be obtained 1n Fig. 10. "From the standpoint of the line are not present, foralthough the equalizer E is convariable impedance L and has already been shown, one of the conditions for equalization is that the terthe line i no i minal impedance at the end of theline opposite to the equalizer must be fixed. The

' same argument holds true for the equalizer E and the line L.

case of such a circuit as that of Fi 9 by interposing a one-Way amplifier, suc as A sion of additional equipment in Fig. 11, between the lines L and L, bridgingif necessary a pure resistance r acrossthe input side of the amplifier so that the in ut circuit of the amplifier will be equiva cut to a'pure resistance. It will be seen at oncethat the conditions for equalization are met so far as the line L is concerned and as the amplifier A has an output impedance of say 500.0hms (the amplifier being similar to the amplifier of the pickup arrangement which constituted the sending termination S ofFig. 2), the conditions for equalization of the line L are alsomet. Such an arrangement has the obvious disadvantage that the amplifier A constitutes an additional piece of equipment for each connection, which might make the .circuit uneconomical.

An alternative proposal has been to interpose between the two lines a resistance artificial line or network N as shown in' Fig. 12. This network may, for example, be the socalled H type of network familiar in .telephone practice and comprising series resistances in each line conductor and a shunt resistance'bridged across the conductor. Obviously the artificial line may be made to have an impedancelooking from the equalizer E e uivalent to a pure resistance, and it may likewise be -made to have an impedance looking from the left-hand end of the line L equivalent to 500 ohms. Under these conditions .the two lines of the over-all circuit will be independently equalized, but the transmission loss is reatly increased due to the loss of the arti cial line N. Not only does the artificial line N involve the provicases it .may introduce so much additional transmission loss that another amplifier may have to be circuit.

In accordance with the present invention .it is proposed to overcome all of these objections'by directly connecting the two lines L and L at their terminals, as shown in Fig. 13, and bridging both equalizers E and E across the terminals of the lines in parallel with each other at the junction point.

.Here the conditions for equalization of both lines are substantially met. From the standpointof the'equalizer E it is clear that the equalizer is connected between the line L with its distant termination S and the equivalent impedance of the equalizer E bridged across the impedance looking into the line L, which, as already stated, is subbut in some provided at some point in the attenuation equalizer at the op associatedi1 and means stantially constant and therefore substantially equivalent to a pure resistance. Likewise, the impedance looking into the equalizer E bridged across the impedance of the and an impedancewhich frequency) as the sum of the transmission characteristics of the two equalized circuits taken separately. The measurements also show that the transmission equivalent of the over-all circuit is actually somewhat less than the sum of the equivalents of the two equalized circuits taken separately, this this being due to the fact that a better impedance match exists when the circuits are connected together at the equalizer point than exists when the circuits are equalized separately and. terminated in pure resistance. V f

It will be obvious that the general principles herein disclosed may be embodied in many other organizations widely different from those illustrated without departing from:.the spirit of the invention as defined in the following claims.

What is claimed is: 1. A transmission system comprising a 'pair of transmission lines,- each terminated at one end in a terminal impedance, an attenuation equalizer for each line connectedthereto at the opposite end, and means to directly connect the ends of said lines at which the attenuation equalizers are located. I Y

2. In a transmission system, a pair of transmission lines each terminated at one end in a terminal impedance, the electrical characteristics of each line being such that the attenuation varies. with frequency, an posite end of each linehaving such electrical characteristics that its attenuation varies with frequency in a manner substantially complementary to that of the line. with which it is to directly connect the ends of t e lines at which the attenuation equalizers are located.

3. In a transmission system, a pair df transmissidn lines each terminated at one end in a terminal impedance,- sin-attenuation equalizer for each line connectedthereto at its opposite end, the attenuation equalizer being so proportioned with res ect to ,the

line with which it associated t at the im-- pedance looking into the combination of the attenuation equalizer and line from the end of the line at which the attenuation equalizer is located will be substantially constant wlth frequency, and means to directly con nect the ends of the lines at which the attenuation equalizers are located so that each attenuation equalizer is in effect connected lid between the substantial equivalent ofthe pure resistance and one end nated at its'opposite end in'a terminal 1mpedance.

4. In a transmission systenua pair of lines each terminated at one end in a terminal impedance, the electrical characteristics of each line being such tenuation varies with frequency, an attenuation equalizer for each line associated therewith at'its opposite end, each attenuation equalizer having electrical characteristics such that its attenuation varies with frequency in a manner substantially complen'ientary to that of the line with which it is associated, and that the impedancelooking into the combination of attenuation equalizer and line from the end of the line at which the attenuation equalizer is located is substantially constant, and means to directly connect the ends of the said lines at which the attenuation equalizers are located whereby each attenuation equalizer is in effect connected between the substantial equivalent of a pure resistance and one end tral switching station,

ot a line termithat its at-' of a line which is terminated at its opposite end in a terminal impedance.

5. A signaling system comprising a centransmission lines extending from said switching station to distant points, terminal impedances for the lines at the distant points, an attenuation equalizer associated with each line at the central switching station and so related to the line that when it is connected between the end of the line and a substantially pure resistance the combination of the line and attenuation equalizer will have substantially uniform attenuation over a range of frequencies to be transmitted, the electrical characteristics of each line and its associated attenuation equalizer being such that the impedance of the combination consisting of attenuation equalizer and line will be substantially constant with frequency over the range to be transmitted, and means at the central switching station for directly connecting the terminals of said lines in pairs so that the attenuation equalizer for each line of a pair is in effect connected between the line and an impedance which is substantially constant with frequency.

In testimony whereof we have signed our names to this specification this 26th day of March, 1926.

MAURICE E. STRIEBY. CHARLES H. FETTER. 

